Today:
Atoms to
Ecosystems II
Kalyn Shea Owens
Ann Murkowski
Vapor Pressure
1. Introductions
2. Group Discussion – Goals for
this quarter?
y
3. Syllabus
4. Colligative Properties
5. Freezing Point Depression
Activity
Vapor Pressure
Vapor Pressure Defined
Every liquid in a closed
container gives off vapor
until a certain p
pressure is
reached - the liquid’s
vapor pressure.
Volatility Defined
• A liquid in an open container will
evaporate
• As vapor moves away, the liquid releases
more molecules into the vapor phase to try
to build up to the vapor pressure
• Liquids with high vapor pressure
evaporate more quickly - they are volatile
1
Boiling Point
What is Boiling?
• A liquid boils when the vapor pressure equals
the atmospheric pressure
• Normal boiling point -> when vp is 760 torr.
We list normal bp
p values in textbooks
• Actual boiling point -> when the liquid has a
vp equal to the external atmospheric pressure
• Water boils at temperature lower than 100ºC
on mountains - it never reaches 100ºC
• Water boils at higher temp in pressure cooker
Which has the higher Boiling Point?
Which has the higher vapor pressure?
Lowering the Vapor Pressure
09_10.jpg
Why do you think
the liquid levels in
the two beakers
change over time?
Animation
Distilled Water Seawater
Boiling Point: the temperature at
which vp = 760 torr
Lowering the Vapor Pressure
• Addition of a nonvolatile solute lowers the
vapor pressure of the solvent
• Extent of lowering depends on concentration
of solute, described by Raoult’s Law
• Raoult’s Law – The equilibrium vapor
pressure of the solvent over the solution is
directly proportional to the mole fraction of the
solvent in the solution
Psolution = XsolventP°solvent
Seawater
2
Lowering the Vapor Pressure
• Glycerin (C3H8O3) is a nonvolatile nonelectrolyte with a density of 1.26 g/ml at 25
°C. Calculate the vapor pressure at 25 °C of a
solution made by adding 50.0 ml of glycerin
to 500.0 ml of water. (The vapor pressure of
pure water at 25 °C is 23
23.8
8 torr
torr.))
Conceptual Question
• A 0.100 m solution of which one of the
following solutes in water will have the
lowest vapor pressure?
a.
a
b.
c.
d.
e.
KClO4
Ca(ClO4)2
Al(ClO4)3
Sucrose
NaCl
Phase Diagrams – From Ch. 12
A graphical way to show the equilibria
between different phases of a
substance.
Things to look for:
critical point
triple point ( three phases)
how bp (and mp) varies with
pressure
Describe
what’s
taking
place at
each point
p
from 1-5,
then from
6-9.
3
At the Normal Boiling Point of a pure liquid,
vapor pressure of solution is < 1 atm.
What
does this
say about
the boiling
point of
the
solution?
Boiling-Point Elevation
The boiling point of a solution is higher than
that of a pure liquid
Animation
Freezing
Point
Depression
The freezing
point of a
solution is
lower than that
of a pure
solvent
Calculations
• The concentration of table sugar in a
sucrose solution is 0.50 m. Calculate
the freezing-point for this solution. (Kf of
water is 1
1.86
86 °C/m)
C/m)
Boiling-Point Elevation and
Freezing Point Depression
• Boiling Point Elevation
– directly proportional to the molality of the
solute
– ΔTb = Kb msolute
l t
– Kb is a constant, m is molality
• Freezing Point Depression
– ΔTf = Kf msolute
– Kf is a constant, m is molality
More Calculations
• An environmentally concerned graduate
student adds the more earth-friendly coolant,
propylene glycol (C3H3O2, density = 1.038) to
her car’s radiator, resulting in a final
composition of her radiator fluid equaling 30%
(v/v) ethylene glycol (C2H6O2, d=1.114
d=1 114 g/ml)
and 20% (v/v) propylene glycol in water. At
what temperature will her engine “boil over”?
(Kb = 0.50 °C/m)
4
Conceptual Question
• Which of the following will have the
lowest freezing point?
a.
b
b.
c.
d.
e.
Pure H2O
Aqueous glucose (0
(0.60
60 m)
Aqueous sucrose (0.60 m)
Aqueous FeI3 (0.24 m)
Aqueous KF (0.50 m)
5